Management method of rights of a content encrypted and stored in a personal digital recorder

ABSTRACT

The aim of the present invention is to propose a storage method of an event encrypted by control words (CW) that guarantees the access to this event whichever the moment, even if certain modifications in the designation of the identifiers of these events occur between the moment of storage and the moment of viewing.  
     This aim is achieved by a storage method of such an event in the reception and decryption unit (STB) connected to a security unit (SC), said control words (CW) and the necessary rights being contained in control messages (ECM), characterised in that it comprises the following stages:  
     storing the encrypted event as well as the control messages (ECM) in the storage unit,  
     transmitting the control messages (ECM) to the security unit (SC),  
     verifying if the access rights to this event are contained in the security unit (SC),  
     if this is the case, determining a receipt (Q) of all or part of the control message (ECM) thanks to a secret key (K) contained in the security unit (SC) and specific to each security unit,  
     storing this receipt (Q) in the storage unit.

[0001] The present application concerns the field of receivers/decoders of conditional access services, particularly of receivers that have a storage unit such as hard disks.

[0002] The technological evolution in the field of storage capacities and the speed of the magnetic disks (hard disks) has made it possible to store the content of a transmitted video to make it accessible off-line to a user.

[0003] Such recorders are known with the brand ReplayTV® or Tivo® and propose storages of several tens of hours of digital transmission. These recorders are however not directly integrated in the receivers/decoders of conditional access services; particularly, the content is stored without specific protection on the disk, which makes it impossible to collect the authors' royalties associated to the content in the case where the disk would then be duplicated with commercial redistribution ends.

[0004] Inversely, in a digital pay television system, the digital flux transmitted to these receivers is encrypted in order to be able to control the use and to define the conditions for such a use. This encryption is carried out thanks to control words that are changed at a regular interval (typically between 5 and 30 seconds) so as to dissuade from any attack trying to recover such a control word.

[0005] According to a particular embodiment, the control words are changed at much more longer intervals, which means that for a considered event, it is encrypted by one single control word.

[0006] For the receiver to be able to decrypt the flux encrypted by these control words, the latter are sent to it independently of the flux in control messages (ECM) encrypted by a key specific to the transmission system between the operating system (CAS) and the security module of the user unit. In fact, the security operations are performed in a security unit (SC) that is generally in the form of a smart card, reputed inviolable. This unit can either be of a moveable type or directly integrated in the receiver.

[0007] During the decryption of a control message (ECM), the presence of the right to access the considered flux is verified in the security unit (SC). This right can be administered by authorisation messages (EMM) that charge such a right in the security unit (SC). Other possibilities are equally possible, such as the sending of decryption keys.

[0008] In this description, we will use the name “event” for a content of video, audio (for example MP3) or data (a game programme for example) that is encrypted according to the known method of control words, each event being able to be encrypted by one or several control words, each having a determined duration of validity.

[0009] The accountancy of use of such events is based today on the principle of subscription, of the purchase of events, or of the payment per time unit.

[0010] The subscription allows to define a right associated to one or several transmission channels of these events and allows the user to obtain these events in plaintext if the right is present in its security unit.

[0011] At the same time, it is possible to define rights that are specific to an event, such as a film or a football match. The user can acquire this right (purchase for example) and this event will be specifically administered by this right. This method is known as pay-per-view (PPV).

[0012] As for the payment per time unit, the security unit comprises a credit that is flow-rated depending on the real consumption of the user. In this way, for example, a unit will be flow-rated every minute on its credit whichever the channel or event regarded. It is possible according to the technical implementations to vary the accountancy unit, either in the duration or in the value of the assigned time, even combining these two parameters to adapt the invoicing to the type of transmitted event.

[0013] A control message (ECM) does not only contain the control word, but also the conditions for this word to be resent to the receiver/decoder. During the decryption of the control words, the presence in the security unit of a right associated with the announced conditions of access in the message will be verified.

[0014] The control word is returned to the user unit only when the comparison is positive. This control word is contained in a control message ECM that is encrypted by a transmission key TK.

[0015] For the right to be present in the security unit, it is generally charged in this unit by a right management message (EMM), which for security reasons is generally encrypted by a different key called right key (KR).

[0016] According to a known form of pay television transmission, the following three elements are necessary for decrypting an event at a given moment:

[0017] the event encrypted by one or several control words (CW),

[0018] the control message or messages ECM containing the control words (CW) and the access conditions (AC),

[0019] the corresponding right stored in the security unit allowing to verify said access conditions.

[0020] According to a known layout, the encrypted event that is stored in a storage unit such as a hard disk is accompanied at least by one or several control messages ECM.

[0021] Due to the fact that the decryption a posteriori of the ECM messages can be a problem, particularly because of the changing of the transmission key, a first solution is proposed in the document EP 0 912 052, which solution implies the decryption of these messages in the security unit and the re-encryption before the storage on the disk.

[0022] This solution solves the problem of the working life of the transmission key, but charges greatly the security unit at the moment of the recording, without knowing if the recorded content will one day be used. Furthermore, one of the fundamental rules of the security system is to return the control words to the user unit only if the rights exist. In this case, it is very probable that these rights do not exist if we consider a purchase per event. The right will be acquired during the purchase that can be done much later, when the user decides to view this event.

[0023] This document EP 0 912 052 does not solve the problem of access to the right as at the moment of the purchase, the right message EMM has to be always transmitted so that it is charged in the security unit.

[0024] In this way, the solution described in this document is only applicable for transmitted events for which the right is already present in the security unit in order to authorise the decryption and the re-encryption of the ECM.

[0025] Another aspect is the conservation of the rights of a holder. Let us take for example when a holder A has reception rights of the channels M, N, P. He/she then has the right to view these channels and thus to record and view at will the events that are in his/her storage unit. With each use of such an event, the security unit will be required to decrypt the messages ECM and to return the control words. It is then important that the rights linked to this event are present in the security unit.

[0026] In the case of an event obtained by a subscription, the identification of this event is associated to the subscription channel, for example M. Thus all the events that bear the identifier M are authorised and the control words are returned to the decoder.

[0027] These rights are then associated to a particular channel defined by an identifier such as M. When the subscriber cancels his/her subscription, or modifies it for other channels, it results that the events recorded in the storage unit will be inaccessible because the security unit will refuse to resend the control words, the corresponding right no longer being present.

[0028] This situation can also take place if a new identifier is attributed to the channel M. It is thus possible that the reorganisation of the channels attributes this channel the identifier J4 instead of M. From the point of view of the transmission rights, the security unit is informed in useful time and the user records no disagreement.

[0029] On the contrary, the consequences for a recorded event are more dramatic. This re-assignment will simply produce that the recorded event will be inaccessible, because the corresponding right is no longer present in the security unit.

[0030] The objective of the present invention is to propose a storage method of an event encrypted by control words (CW) that guarantees the access to this event at whichever moment, even if certain modifications occurred between the moment of storage and the moment of viewing.

[0031] This objective is achieved by a storage method of an event encrypted by one or several control words (CW) in a reception and decryption unit (STB) connected to a security unit (SC), these control words (CW) and the necessary rights being contained in control messages (ECM), characterised in that it comprises the following stages:

[0032] storing the encrypted event and the control messages (ECM) in the storage unit,

[0033] transmitting the control messages (ECM) to the security unit (SC),

[0034] verifying if the access rights to this event are contained in the security unit (SC),

[0035] determining a receipt (Q) of all or part of the control message (ECM) thanks to a secret key (K) contained in the security unit (SC) and specific to each security unit,

[0036] storing this receipt (Q) in the storage unit.

[0037] According to a first embodiment of the invention, this receipt is constituted by an signature of all or part of the control message and constitutes a super-right that will then allow the subsequent use of the event, to verify prioritarily this receipt before verifying the usual rights in the security unit. The presence of this receipt, once recognised by a given control message, allows to ignore the access conditions.

[0038] According to a second embodiment of the invention, during the generation of the receipt, apart from the signature, a new part is added that describes how to process this control message when it is presented to the security unit. This condition can be to ignore all the conditions announced in this message (which takes us back to the previous solution) or to announce other conditions such as to dispose of a right of reproduction or to define a window in time to authorise such a reproduction.

[0039] To determine the signature, we will preferably take a part that is not changed for the entire event. In fact, the message ECM comprises schematically two parts:

[0040] a. the control word for the decryption (or the even and odd words)

[0041] b. the right necessary to return this control word.

[0042] This receipt allows to mark a control message and thus to add other information destined to the processing in reproduction mode. The objective is then to identify a control message in an indubitable way. In practice, we see that the part b, that is to say the necessary right, changes less frequently than the control word. This is why we will preferably choose this part to calculate the signature. Nevertheless, it is not excluded to determine the signature on the control word, or the group of the two parts.

[0043] For the calculation of this signature, we determine a unique image of the part considered by a one-way function and without collision with these data. It is admitted that there does not exist a different group of data that give the same result as this function. This image H is produced by a function of the Hash type. The used algorithm can be of the SHA-1 or MD5 type, and this image expresses the group of data in a unique way.

[0044] The following operation consists in encrypting these data thanks to an encryption key K.

[0045] Before the encryption operation, by the key K, it is possible to add a data field CD that describes the new access conditions. The group of these data (H and CD) constituting the receipt is then encrypted by the signature key K.

[0046] In the spirit of the invention, the term receipt means that it is determined by a group of data that are representative of the access conditions (for example in the most simple case) and unique for one security unit concerned thanks to the encryption key K. According to one embodiment, it is possible to encrypt directly the access conditions of the control message ECM by this key without passing through the Hash operation. According to another embodiment, it is possible to determine this unique image (Hash function) on the access conditions and then to decrypt this image by a first key K1, to add the new access conditions CD, and to encrypt it all with the same key K1, or with a second key K2.

[0047] The invention will be better understood with the help of the following detailed description that makes reference to the annexed drawings that are given as a non-limiting example, namely:

[0048]FIG. 1 describes a user unit STB with a storage unit,

[0049]FIG. 2 describes the group of stored data in the storage unit,

[0050]FIG. 3 describes the structure of a control message ECM.

[0051] The decoder STB illustrated in FIG. 1 receives the input data in encrypted form. These data are stored in the storage unit HD and comprise notably the considered event EV and the control messages ECM.

[0052] In this way, according to the invention, these two groups of data are accompanied by a new group that is illustrated in FIG. 2 by the block of receipt Q.

[0053] The size of the different blocks is given here as an example. We can nevertheless consider that the event EV occupies the biggest part, the control messages ECM a small part, and according to an embodiment, one single receipt is enough for the group of these data.

[0054] In fact, if this signature is carried out on the part of the access conditions of the control message, it will not vary for all the considered event.

[0055] On FIG. 3 is illustrated the structure of a control message ECM. This message contains, as described previously, the control word CW and the access conditions. These conditions are divided in two parts, one part specific to the transmission conditions ACB, and a part specific to the reproduction conditions ACR. This message also comprises a time mark TP.

[0056] Amongst these conditions we can find:

[0057] the number of the channel (or services), particularly useful for the subscription,

[0058] the theme of the event (for example, sports, news, adult),

[0059] the level (prime time, afternoon, re-transmission),

[0060] a number for impulsive purchase.

[0061] The duplication of the conditions opens possibilities to the management of the event during the reproduction. The receipt Q can mean that it is necessary to conform simply with the conditions of reproduction or it can mean, on the contrary, to ignore these conditions.

[0062] Let us take the example of a geographic blocking function. This function allows to block the reception of a sports event for example in the 30 km surrounding the stadium. If this blocking has a sense at the moment of the event, on the contrary, a few days later it has no reason of being.

[0063] In the transmission conditions ACB we will find the conditions of the blocking by sector of security unit numbers or by postal code. Regarding the conditions of reproduction ACR, we will find a simple authorisation for all from a certain date (as long as the other conditions such as subscription are fulfilled).

[0064] During the reproduction, the receipt Q is charged first and decrypted by the secret key K, to obtain the signature SGN and the new access conditions CD.

[0065] The signature SGN is then conserved in the memory of the security unit with the new CD conditions. When we present a control message ECM to the security unit, it determines by the Hash function a unique image H′ on the part of the rights AC according to this example and compares this H′ value with the signature SGN.

[0066] If the two values are identical, the security unit applies the conditions announced in the CD conditions part of the receipt. If this CD condition means “free access”, this allows to no longer verify the conditions contained in the control message ECM and thus allows to free itself of all the structural changes of the transmission channels.

[0067] According to another embodiment, the new condition CD resends the reproduction conditions ACR. In these conditions, we do not find any more references to the channels or other elements that could vary in time (structural conditions), but conditions on the time during which this access is agreed or a number of times. We understand in this case that the access conditions linked to a subscription or others have been verified during the formation of the receipt.

[0068] The receipt can be evolutionary. In certain cases, it can be interesting to store a new receipt more favourable than the old one. This is the case notably of an impulsive purchase. In this case, a first receipt is generated during the storage without the user having purchased this event.

[0069] The conditions contained in this receipt will resend the conditions contained in the control message ECM.

[0070] At the moment when the user decides to buy this event, a new receipt is generated that opens the way to a use without reserve of this event if the conditions are defined as such. This receipt is then transmitted to the storage unit to replace the old one. 

1. A storage method of an event encrypted by one or several control words (CW) in a reception and decryption unit (STB) connected to a security unit (SC), said control words (CW) and the necessary rights for the access to this event being contained in control messages (ECM), characterised in that it comprises the following steps: storing the encrypted event as well as the control message or messages (ECM) in the storage unit, transmitting the control messages (ECM) to the security unit (SC), verifying if the access rights to this event are contained in the security unit (SC), determining a receipt (Q) comprising an signature (SGN) of all or part of the control message (ECM) based on a secret key (K) contained in the security unit (SC) and specific to each security unit, storing this receipt (Q) in the storage unit.
 2. A method according to claim 1, characterised in that the receipt (Q) is only calculated if the access rights are present in the security unit.
 3. A method according to claim 1, characterised in that the receipt (Q) comprises also a conditional part (CD) describing the new conditions independent of the structural configuration of the transmission of the event. 